As an example of a multi-beam satellite communication system in the past that covers a communication area with a plurality of beams, a satellite communication system described in Patent Literature 1 is explained. The satellite communication system described in Patent Literature 1 includes a satellite and ground stations connected to a ground network line. The satellite forms a plurality of beams. When areas in which radio terminals and the satellite can communicate using the beams formed by the satellite are referred to as beam areas, all lines that communicate with the beam areas are referred to as user link radio lines.
In this communication system, a frequency band used by the user link radio lines is the same in all the beam areas (a frequency f1). However, a frequency band used in feeder link radio lines, which are radio lines between the satellite and the ground stations, is a frequency different from f1. The satellite is a stationary satellite or an orbit satellite that orbits the Earth.
First, a flow of communication of a forward link (a direction in which signals are transmitted from a user connected to the ground network line to the radio terminals through the satellite) is explained. The satellite receives forward link signals from the ground network line via the ground stations using the feeder link radio lines. Further, after splitting and extracting the received forward link signals from the ground network line, the satellite combines the forward link signals in beam unit while distributing the forward link signals to the beam areas according to control command information from the ground stations and transmits the forward link signals to the beam areas using the user link radio lines. According to the signal processing flow, the radio terminals present in the beam areas can receive signals transmitted from users of the ground network line.
A flow of communication in a return link (a direction in which signals are transmitted from the radio terminals to the ground network line through the satellite) is explained. The satellite receives return link signals from the radio terminals in the beam areas using the user link radio lines. Further, after splitting and extracting the received return link signals from the beam areas according to control command information from the ground stations, the satellite combines signals from a plurality of beams and transmits the signals to the ground stations using the feeder link radio lines. The ground stations split and extract reception signals from the satellite and transmit the reception signals to the ground network line. According to the signal processing flow, the signals transmitted from the radio terminals in the beam areas can be transmitted to the users of the ground network line.
The satellite of the satellite communication system in the past realizes multi-beam transmission and reception on the user link (user link radio line) side using a digital beam forming technology. Specifically, the satellite includes a user link side transceiver. The user link side transceiver includes reception array antenna elements including N (N is a natural number) array antenna elements, low-noise amplifiers (LNAs), down-converters (D/Cs), reception analog filters, AD (Analog to Digital) converters, a reception DBF (Digital Beam Forming) network, reception-DBF control units, reception FBs (Filter Banks), a reception-FB control unit, a transmission-FB control unit, transmission FBs, a transmission-DBF control unit, a transmission DBF network, DA converters, transmission analog filters, up-converters (U/Cs), power amplifiers (PAs), and transmission array antenna elements.
The user link side transceiver receives, with a reception array antenna, signals transmitted from the radio terminals in the beam areas. In some case, the reception array antenna receives signals from the beam areas via reflecting mirrors. Each of the N LNAs amplifies a reception signal received by the reception array antenna corresponding to one array element. Each of the N D/Cs frequency-converts a reception signal after amplification corresponding thereto into a direct current (DC) or an intermediate (IF) frequency. Each of the N reception analog filters extracts a desired system band signal from the reception signal after the frequency conversion corresponding thereto. Each of the N A/D converters samples the signal after passing the reception analog filter corresponding thereto and converts the signal into a digital signal.
The reception-DBF control unit calculates, based on control command information transmitted from the ground stations through the feeder link radio lines (information concerning, for example, a beam radiation direction calculated from the location and the posture of the satellite) and A/D-converted digital signals, weight values for forming reception antenna patterns directed in an arriving direction of a desired signal and outputs a result of the calculation to the reception DBF network. After multiplying together weight values corresponding to L (L is a natural number) digital signals among N digital signals and performing amplification and phase control, the reception DBF network adds up all the weight values to form a first reception antenna pattern and outputs a result of the addition as a first reception beam signal.
Similarly, the reception DBF network multiplies together weight values corresponding to other L digital signals and adds up all the weight values to form a second reception antenna pattern and outputs a result of the addition as a second reception beam signal. In this way, the reception DBF network outputs M (M is a natural number) reception beam signals in total from the first reception beam signal to an Mth reception beam signal.
The reception-FB control unit outputs, based on control command information transmitted through the feeder link radio lines, frequency division instruction information indicating division content of the reception beam signals to M reception FBs. Each of the M reception FBs splits, based on the frequency division instruction information from the reception-FB control unit, the reception beam signal corresponding thereto into a plurality of signals.
The reception FB and the transmission FB explained later can be realized by, for example, a configuration described in Non-Patent Literature 1. Non-Patent Literature 1 describes a configuration for realizing {division into two, division into four, division into eight} of a band of an input signal. The reception FB includes first to seventh seven two-channel filter banks and a selecting unit. Each of the two-channel filter banks includes a high-frequency side decimeter that down-samples sampling speed to a half after dividing a frequency band of an input signal into two and extracting a divided higher frequency component and a low-frequency side decimeter that down-samples sampling speed to a half after extracting a divided lower frequency component.
A signal input to the reception FB is first input to a first two-channel filter bank. An output of the high-frequency side decimeter and an output of the low-frequency side decimeter of the first two-channel filter bank are respectively input to the second two-channel filter bank and the third two-channel filter bank. An output of the high-frequency side decimeter and an output of the low-frequency side decimeter of the second two-channel filter bank are respectively input to the fourth two-channel filter bank and the fifth two-channel filter bank. An output of the high-frequency side decimeter and an output of the low-frequency side decimeter of the third two-channel filter bank are respectively input to the sixth two-channel filter bank and the seventh two-channel filter bank. Outputs of the first to seventh two-channel filter banks are input to the selecting unit.
For example, in the case of an input signal including four frequency bands F1 to F4 (F1<F2<F3<F4, when frequency band width of F1 is 1, frequency band widths of F2, F3, and F4 are respectively 1, 2, and 4), the selecting unit can obtain a signal in the frequency band F1 by selecting the output of the low-frequency side decimeter of the seventh two-channel filter bank. The selecting unit can obtain a signal in the frequency band F2 by selecting the output of the high-frequency side decimeter of the seventh two-channel filter bank. The selecting unit can obtain a signal in the frequency band F3 by selecting the output of the high-frequency side decimeter of the third two-channel filter bank. The selecting unit can obtain a signal in the frequency band F4 by selecting the output of the high-frequency side decimeter of the first two-channel filter bank.
The selecting unit discards frequency components other than the frequency bands used in this satellite system without selecting the frequency components. For example, when the signal in the frequency band F4 is not a signal of this satellite system (e.g., in the case of an interference wave or signals of other systems), the selecting unit discards the output of the high-frequency side decimeter of the first two-channel filter bank without selecting the output.
Split signals selected and output by the selecting unit are combined by the satellite together with signals obtained by splitting other reception beam signals and are transmitted to the ground stations using the feeder link radio lines.
A transmission operation is explained below. The transmission FB corresponding to each beam combines, based on frequency combining instruction information from the transmission-FB control unit, signals transmitted from the feeder link radio lines into one transmission beam signal. In other words, M transmission beam signals are output from M transmission FBs. The transmission DBF network multiplies L′ copied predetermined transmission beam signals with L′ weight values instructed by the transmission-DBF control unit. When this processing is executed on each of the M transmission beam signals, L′×M signals are obtained. The transmission DBF network outputs N (<L′×M) DBF transmission signals by combining the L′×M signals as appropriate by sharing the transmission array elements.
Each of the N D/A converters converts the DBF transmission signal corresponding thereto from a digital signal into an analog signal. Each of the N transmission analog filters removes an image component from the analog signal corresponding thereto. Each of the N U/Cs frequency-converts the signal (the analog DBF signal) after the image removal corresponding thereto into a radio frequency different from a frequency on a reception side.
Each of the N PAs amplifies the analog DBF signal converted into the radio frequency corresponding thereto. The transmission array antenna outputs amplified analog DBF signals to the space. The transmission array antenna can output the analog DBF signals to the space via the reflecting mirrors.
To realize frequency sharing with other systems, the satellite of the satellite communication system described in Patent Document 1 detects a signal (=an interference wave) from the other systems. When a signal from the other systems is detected, the satellite performs prevention of influence of the interference through null formation of an antenna pattern in an arriving direction of the signal. Concerning a method of estimating an arriving direction of a signal source, various methods are already established. For example, a beam former method and a multiple signal separating method (MUSIC) are representative methods.
The reception-DBF control unit performs specific signal processing for interference prevention. The reception-DBF control unit analyzes a signal source of an arriving signal. When the reception-DBF control unit determines that the signal is not a signal of the own satellite communication system, the reception-DBF control unit determines that the signal is an interference wave, calculates a weight value to perform null formation of an antenna pattern in an interference direction, and outputs the weight value to the reception DBF network. The reception DBF network performs the null formation of the antenna pattern in the interference direction using this weight value and performs interference reduction to a degree not affecting communication.
Citation List
Patent Literature
Patent Literature 1: Japanese Patent Application Laid-Open No. H10-145260
Non Patent Literature
Non Patent Literature 1: “Multi-Rate Signal Processing”, Hitoshi Kiya, Shokodo, pp. 94, FIG. 6.5 (a), (b)
Non Patent Literature 2: “Ground and Satellite Shared Mobile Communication System for Safety and Relief”, Institute of Electronics, Information and Communication Engineers, 2008, General Meeting, BP-1-3